The invention described herein arose in the course of, or under, Contract No. DE-AC03-SF00098 between the United States Department of Energy and the University of California for the operation of the Lawrence Berkeley Laboratory.
1. Field of the Invention
This invention relates to a method of making a multileaf collimator for radiation beams such as, for example, heavy charged particle beams used in the treatment of cancer by radiation. More particularly, this invention relates to a method for minimizing the width of the leaves when making sawtooth joints between leaves of a multileaf collimator which provides an adjustable aperture while still preventing leakage of radiation between the leaves.
2. Description of the Related Art
In the treatment of cancer by radiation, a shield is conventionally placed between the patient and the radiation source to provide limited exposure of the patient to the radiation beam. The size of the shield varies with the size of the area to be irradiated, as well as the size of the patient. For this reason, the use of an adjustable collimator, i.e., a collimator with adjustable leaves, has been previously proposed. However, the use of an adjustable collimator introduces the additional problem of possible radiation leakage through the cracks or joints between adjacent leaves. In the published Proceedings of the NIRS International workshop on Heavy Charged Particle Therapy and Related Subjects, Jul. 4-5, 1991 National Institute of Radiological Sciences, on pages 115-116, there is described a multileaf collimator having a number of independently adjustable leaves on each side of the collimator, as shown in FIG. 1. Such a collimator is rotatable 90 degrees around the central axis of the radiation beam, and the leaves can be operated horizontally, vertically, or at any angle therebetween, thus allowing the collimation of almost all field shapes encountered in radiation therapy.
In the adjustable collimator generally shown at 2 in FIG. 1, a first stack of independently adjustable leaves 4a-4p is positioned on one side of the beam path, and a second opposed set of independently adjustable leaves 6a-6p is positioned on the opposite side of the beam path. Each leaf slides along a track (not shown) and the motion of each leaf is independently controlled by its own DC motor means 8a-8p and 10a-10p. The result, as shown in FIG. 1, is an adjustable opening which can be tailored to the individual size of the patient and treatment area on that patient. While such an adjustable collimator greatly increases the flexibility of patterning the opening to various sizes and shapes, it will be immediately apparent that such adjustability creates a potential problem of radiation leakage through the joints between adjacent adjustable leaves. In the NIRS Proceedings publication, the authors state that the adjacent surfaces of the leaves may be shaped in a sawtooth pattern, to inhibit particles from leaking through such joints or cracks between the leaves. The authors further mention that such a corrugated geometry has been tested with helium and neon ion beams, with no film detection of particle leakage through the cracks. However, the use of such a sawtooth configuration for the mating surfaces of adjacent leaves, as illustrated in FIG. 2, can greatly increase the required total width of each leaf, depending upon the configuration of the sawtooth edge and the gap G between the respective sawtooth surfaces of adjacent leaves. In FIG. 2, leaf 4a is shown with sawtooth edges 12a and 12b, 14a and 14b, 16a and 16b, and 18a and 18b which face mating sawtooth edges 22a and 22b, 24a and 24b, 26a and 26b, and 28a and 28b in adjacent leaf 4b in the same stack. When a radiation beam, as shown at beam path R, of a given intensity or energy, penetrates the solid portion of leaves 4a or 4b, the normally penetrates to some distance, as shown as point 20, which must be less than the total thickness of leaves 4a and 4b for the shielding to be effective. However, the width of leaves 4a and 4b must be wider than this penetration depth of beam path R if beams penetrating the stack of leaves at a joint, such as shown at beam paths R.sub.1 and R.sub.2 in FIG. 2, are also to be completely shielded, because only portions of the R.sub.1 and R.sub.2 beam paths pass through shielding material of leaves 4a and 4b, with the remainder of the R.sub.1 and R.sub.2, beam paths passing through air, as shown by the dotted lines, i.e., with no attenuation of the beams. It will also be noted, by examining FIG. 2, that the respective beam paths denoted in the Figure as R.sub.1 and R.sub.2, are drawn to coincide with the tips of the respective saw teeth to illustrate the area, between lines R.sub.1 and R.sub.2, wherein the beam path through air is maximized and the beam path through the shielding material of the leaves is minimized.
As shown in FIG. 2, for beam R.sub.1 or R.sub.2 to travel the same distance, respectively, through leaves 4a or 4b as beam R travels through leaf 4b before it is completely absorbed or attenuated at 20, can require a considerable addition to the total width of the leaves. In the illustration of FIG. 2, this added width could be almost twice the minimum width required for absorption of the beam at beam path R passing through the solid portion of the leaf, i.e., not at a joint. As a result, in this prior art construction, beam paths R.sub.1 and R.sub.2 are shown as passing completely through leaves 4a and 4b at the joint, i.e., penetrating through the collimator, a highly undesirable condition.
It will also be apparent, for example, by examining the respective joints formed by closed leaves 4a-6a, 4b-6b, and 4p-6p, that the same problem of penetration of the particle beam can result at the intersection of the respective opposing leaves in the two stacks when the leaves are in a closed position, i.e., abutting one another. While such end edges may also be formed with mating sawtooth surfaces, the same problem of maximizing the pathway of the particle beam through the shielding material occurs, resulting in the same need for added width of the leaves.
The addition of further width to the leaves, however, to prevent such unwanted penetration, either between adjacent leaves in the same stack, or through the joint created between opposing leaves in the closed position, results in added weight for each leaf. Since there must be a number of such leaves in order to provide sufficient resolution of the pattern to be outlined by the adjustable leaves, such added width of each individual leaf can add considerable weight to the overall structure of the collimator. When it is considered further that it is desirable to be able to rotate the collimator so that the adjustment of the leaves may, in essence be along two axes, rather than only one (i.e., in the plane normal to the beam path), the weight lo considerations are even further impacted by the width of the individual leaves, since added weight requires the provision of a more rugged rotational means as well.
It would, therefore, be desirable to provide an optimization of the width of the individual leaves of the collimator without, however, jeopardizing the penetration of the particle beam through the collimator at the joints between adjacent or opposing leaves.